U.S. patent number 11,431,070 [Application Number 17/258,366] was granted by the patent office on 2022-08-30 for flexible substrate.
This patent grant is currently assigned to Mitsubishi Electric Corporation. The grantee listed for this patent is Mitsubishi Electric Corporation. Invention is credited to Shin Chaki.
United States Patent |
11,431,070 |
Chaki |
August 30, 2022 |
Flexible substrate
Abstract
A flexible substrate (1) is bent at a bending part (2). A
dielectric plate (3) has first and second main surfaces opposite to
each other. A high-frequency signal line (4) is provided on the
first main surface of the dielectric plate (3). A ground conductor
(5) is provided on the second main surface of the dielectric plate
(3). The high-frequency signal line (4) and the ground conductor
(5) form a micro strip line. A local absent part (6) facing the
high-frequency signal line (4) is provided on the ground conductor
(5) only at the bending part (2).
Inventors: |
Chaki; Shin (Nagasaki,
JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Mitsubishi Electric Corporation |
Tokyo |
N/A |
JP |
|
|
Assignee: |
Mitsubishi Electric Corporation
(Tokyo, JP)
|
Family
ID: |
1000006531950 |
Appl.
No.: |
17/258,366 |
Filed: |
November 22, 2018 |
PCT
Filed: |
November 22, 2018 |
PCT No.: |
PCT/JP2018/043238 |
371(c)(1),(2),(4) Date: |
January 06, 2021 |
PCT
Pub. No.: |
WO2020/105181 |
PCT
Pub. Date: |
May 28, 2020 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20210296750 A1 |
Sep 23, 2021 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H05K
1/0277 (20130101); H01P 3/08 (20130101); H05K
1/0237 (20130101) |
Current International
Class: |
H01P
3/08 (20060101); H05K 1/02 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
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2574155 |
|
Mar 2013 |
|
EP |
|
H04-22075 |
|
Jan 1992 |
|
JP |
|
2000-091801 |
|
Mar 2000 |
|
JP |
|
2007-123740 |
|
May 2007 |
|
JP |
|
2007-129111 |
|
May 2007 |
|
JP |
|
2012-253362 |
|
Dec 2012 |
|
JP |
|
2014-099657 |
|
May 2014 |
|
JP |
|
2013/094471 |
|
Jun 2013 |
|
WO |
|
Other References
International Search Report; Written Opinion; and Notification of
Transmittal of the International Search Report and the Written
Opinion of the International Searching Authority, or the
Declaration issued in PCT/JP2018/043238; dated Feb. 19, 2019. cited
by applicant .
Office Action issued in JP 2019-518327; mailed by the Japanese
Patent Office dated Jun. 25, 2019. cited by applicant .
Office Action issued in JP 2019-518327; mailed by the Japanese
Patent Office dated Sep. 3, 2019. cited by applicant.
|
Primary Examiner: Outten; Samuel S
Attorney, Agent or Firm: Studebaker & Brackett PC
Claims
The invention claimed is:
1. A flexible substrate bent at a bending part comprising: a
dielectric plate having first and second main surfaces opposite to
each other, a first flat part including first portions of the first
and second main surfaces, respectively, and a second flat part
including second portions of the first and second main surfaces,
respectively; a high-frequency signal line provided on the first
main surface of the dielectric plate and extending along a
longitudinal axis; a ground conductor provided on the second main
surface of the dielectric plate; at least one additional ground
conductor provided on at least one side of the high-frequency
signal line at the bending part; and at least one via penetrating
through the dielectric plate to connect the at least one additional
ground conductor to the ground conductor, wherein the
high-frequency signal line and the ground conductor form a micro
strip line, the bending part is defined by a single local absent
part facing the high-frequency signal line and provided on the
ground conductor only at the bending part which directly connects
the first and second flat parts, the absent part has a structure of
a window shape in which the ground conductor is absent and is not
covered with a conductor film, and the at least one via is formed
at a lateral axis that passes through the high-frequency signal
line and the single local absent part.
2. The flexible substrate according to claim 1, wherein a change of
a local characteristic impedance of the bending part relative to a
characteristic impedance of a flat part of the flexible substrate
is compensated.
3. The flexible substrate according to claim 1, wherein a width of
the ground conductor is the same at the bending part as at portions
other than the bending part.
Description
FIELD
The present disclosure relates to a flexible substrate.
BACKGROUND
Downsizing of a system including an optical communication
transmission-reception module has been increasingly requested to
achieve large capacity and speeding-up as well as space saving.
Thus, along with a request for downsizing of a
transmission-reception module package, a space for mounting the
transmission-reception module in the system tends to decrease.
Necessity for size reduction and a steep bending structure of a
flexible substrate used for high-frequency signal input and output
at the transmission-reception module has been increasing as
well.
Various structures have been disclosed to achieve highly accurate
control of characteristic impedance, reduction of unnecessary
radiation, bending easiness, and the like of a flexible substrate
(refer to Patent Literature 1, for example). However, a
conventional flexible substrate is developed to achieve a
characteristic impedance that does not change between a flat state
and a bent state, and does not have an optimum characteristic
impedance while being bent.
CITATION LIST
Patent Literature
[Patent Literature 1] JP 2007-123740 A
SUMMARY
Technical Problem
In a steep bending structure, bending is performed in a small range
as compared to a conventional gradual bending structure. Thus, for
example, when a micro strip line is used as a high-frequency signal
line of a flexible substrate, the thickness of a dielectric changes
due to steep bending, and accordingly, the characteristic impedance
changes as well. This causes discontinuation between the
characteristic impedance of a steep bending part and the
characteristic impedance of a flat part. As a result, efficient
high-frequency signal propagation cannot be achieved due to
high-frequency signal reflection or loss generation at the steep
bending part, which has been a problem. In particular, a passing
loss is large in a case of propagation of a high-frequency signal
at 10 GHz or higher. It is inevitable that the frequency of a
high-frequency signal will be further increased due to a request
for fast and large-capacity information propagation. Thus,
significant influence will be caused by, for example, degradation
of the performance of a high-frequency signal line included in a
flexible substrate due to bending. This influence cannot be ignored
to maintain high performance or achieve potential module
performance at maximum.
The present invention is intended to solve the above-described
problems and obtain a flexible substrate that can solve
discontinuation between the characteristic impedance of a flat part
and the characteristic impedance of a bending part.
Solution to Problem
A flexible substrate according to the present disclosure includes:
a dielectric plate having first and second main surfaces opposite
to each other, a high-frequency signal line provided on the first
main surface of the dielectric plate; and a ground conductor
provided on the second main surface of the dielectric plate,
wherein the high-frequency signal line and the ground conductor
form a micro strip line, and a local absent part facing the
high-frequency signal line is provided on the ground conductor only
at the bending part.
Advantageous Effects of Invention
In the present disclosure, a local absent part facing the
high-frequency signal line is provided on the ground conductor only
at the bending part. Accordingly, capacitance increase due to
proximity of the high-frequency signal line and the ground
conductor at the bending part can be prevented. That is, change of
the local characteristic impedance of the bending part relative to
the characteristic impedance of the flat part of the flexible
substrate is compensated by the absent part. Thus, discontinuation
between the characteristic impedance of the flat part of the
flexible substrate and the characteristic impedance of the bending
part can be prevented. As a result, it is possible to reduce signal
reflection as well as a passing loss, and thus it is possible to
reliably and efficiently propagate a high-frequency signal of an
optical communication transmission-reception module.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view illustrating a flexible substrate
according to Embodiment 1.
FIG. 2 is a cross-sectional view taken along line I-II in FIG.
1.
FIG. 3 is a perspective view illustrating a state in which the
flexible substrate according to Embodiment 1 is bent.
FIG. 4 is a perspective view illustrating a state in which the
flexible substrate according to Embodiment 1 is bent.
FIG. 5 is an enlarged cross-sectional view of a bending part in
FIG. 3.
FIG. 6 is an enlarged cross-sectional view of the flat part and the
bending part of the flexible substrate according to Embodiment
1.
FIG. 7 is an enlarged cross-sectional view of a bending part of a
flexible substrate according to a comparative example.
FIG. 8 is an enlarged cross-sectional view of a bending part of a
flexible substrate according to a comparative example.
FIG. 9 is a perspective view illustrating a flexible substrate
according to Embodiment 2.
FIG. 10 is a cross-sectional view of the flexible substrate in FIG.
9 being bent, which is taken along I-II.
FIG. 11 is a cross-sectional view of the flexible substrate in FIG.
9 being flat, which is taken along III-IV.
FIG. 12 is a perspective view illustrating a flexible substrate
according to Embodiment 3.
FIG. 13 is a cross-sectional view of the flexible substrate in FIG.
12 being bent, which is taken along I-II.
FIG. 14 is a cross-sectional view of the flexible substrate in FIG.
12 being flat, which is taken along III-IV.
FIG. 15 is a perspective view illustrating a modification of the
flexible substrate according to Embodiment 3.
FIG. 16 is a perspective view illustrating a flexible substrate
according to Embodiment 4.
FIG. 17 is a cross-sectional view taken along I-II in FIG. 16.
FIG. 18 is an enlarged cross-sectional view of the bending part of
the flexible substrate according to Embodiment 4 being bent.
FIG. 19 is a perspective view illustrating a flexible substrate
according to Embodiment 5.
DESCRIPTION OF EMBODIMENTS
A flexible substrate according to the embodiments of the present
disclosure will be described with reference to the drawings. The
same components will be denoted by the same symbols, and the
repeated description thereof may be omitted.
Embodiment 1
FIG. 1 is a perspective view illustrating a flexible substrate
according to Embodiment 1. FIG. 2 is a cross-sectional view taken
along line I-II in FIG. 1. This flexible substrate 1 is used to,
for example, electrically connect an optical communication
transmission-reception module package and a higher-level system.
The flexible substrate 1 is bent at a bending part 2 along line
I-II, but FIGS. 1 and 2 illustrate a state in which the flexible
substrate 1 is not bent.
A dielectric plate 3 has first and second main surfaces opposite to
each other. A high-frequency signal line 4 is provided on the first
main surface of the dielectric plate 3. A ground conductor 5 is
provided on the second main surface of the dielectric plate 3. The
high-frequency signal line 4 and the ground conductor 5 form a
micro strip line. A local absent part 6 facing the high-frequency
signal line 4 is provided on the ground conductor 5 only at the
bending part 2. The absent part 6 has a structure of a window shape
in which the ground conductor 5 is absent. When a plurality of
bending parts 2 exists, one absent part 6 is provided for each
bending part 2.
FIGS. 3 and 4 are perspective views illustrating a state in which
the flexible substrate according to Embodiment 1 is bent. FIG. 5 is
an enlarged cross-sectional view of a bending part in FIG. 3. FIG.
3 illustrates a case of bending in a convex shape on the ground
conductor 5 side, and FIG. 4 illustrates a case of bending in a
convex shape on the high-frequency signal line 4 side. In any case,
the thickness of the dielectric plate 3 decreases when the flexible
substrate 1 is bent. The relation d0>d1 holds, where do
represents the thickness of the dielectric plate 3 at a flat part 7
of the flexible substrate 1 and d1 represents the minimum thickness
of the dielectric plate 3 at the bending part 2 of the flexible
substrate 1 being bent at 90.degree..
FIG. 6 is an enlarged cross-sectional view of the flat part and the
bending part of the flexible substrate according to Embodiment 1.
The capacitance of a metal-insulator-metal (MIM) structure is
proportional to the area of a conductor and inversely proportional
to the thickness of a dielectric. Capacitance C0 between the
high-frequency signal line 4 and the ground conductor 5 at the flat
part 7 is C0=.epsilon.S0/d0, where S0 represents the area of a
conductor at flat part 7 or the bending part 2 at which no absent
part 6 exists. Capacitance C1 at the bending part 2 at which no
absent part 6 exists is C1=.epsilon.S0/d1. Since d0>d1, C0<C1
holds. Thus, capacitance of the flat part 7 is different from
capacitance of the bending part 2 at which no absent part 6 exists.
Capacitance C2 at the bending part 2 at which the absent part 6
exists is C2=.epsilon.S1/d1, where S1 represents the area of a
conductor at this bending part 2. Since S1<S0, C0.apprxeq.C2
holds. Thus, capacitance of the flat part 7 is substantially equal
to capacitance of the bending part 2 at which the absent part 6
exists.
The characteristic impedance of the high-frequency signal line 4 at
the flat part 7 is represented by Z0. The characteristic impedance
of the high-frequency signal line 4 at the bending part 2 of the
flexible substrate 1 being bent is represented by Z1 when no absent
part 6 exists or by Z2 when the absent part 6 exists. The
relational expression Z= (L/C) holds, where Z represents the
characteristic impedance, L represents the inductance of the
high-frequency signal line 4, and C represents the capacitance
between the high-frequency signal line 4 and the ground conductor
5. When the above-described capacitance relation is applied to the
relational expression, Z0>Z1 and Z0.apprxeq.Z2 hold. This
indicates that discontinuation between the characteristic impedance
of the flat part 7 and the characteristic impedance of the bending
part 2 in the flexible substrate 1 can be prevented when the absent
part 6 is provided. Optimum values need to be selected as the
dimensions of the absent part 6 in accordance with the change
amount of the thickness of the dielectric plate 3 due to bending,
the relative permittivity of the dielectric plate 3, the bending
angle, the frequency of a propagating high-frequency signal, and
the like.
FIGS. 7 and 8 are enlarged cross-sectional views of a bending part
of a flexible substrate according to a comparative example. FIG. 7
illustrates a case of bending in a convex shape on the ground
conductor 5 side, and FIG. 8 illustrates a case of bending in a
convex shape on the high-frequency signal line 4 side. No absent
part 6 is provided in the comparative example. The ratio d1/d0 is
given by cos .theta.. The relation d1/d0.apprxeq.0.71 holds when
the bending angle is 90.degree. and .theta. is 45.degree..
Characteristic impedances Z1a and Z2b of the bending part 2 in
FIGS. 7 and 8 are Z1a.apprxeq.Z2b.apprxeq.35.5.OMEGA..
Characteristic impedance Z0 of the flat part 7 is normally designed
to be 50.OMEGA. but is 50.OMEGA. or smaller at the bending part 2
of the comparative example. Thus, discontinuation of the
characteristic impedance occurs in the comparative example, and
accordingly, RF reflection occurs, which leads to a large passing
loss.
However, in the present embodiment, the local absent part 6 facing
the high-frequency signal line 4 is provided on the ground
conductor 5 only at the bending part 2. Accordingly, capacitance
increase due to proximity of the high-frequency signal line 4 and
the ground conductor 5 at the bending part 2 can be prevented. That
is, change of the local characteristic impedance of the bending
part 2 relative to the characteristic impedance of the flat part 7
of the flexible substrate 1 is compensated by the absent part 6.
Thus, discontinuation between the characteristic impedance of the
flat part 7 of the flexible substrate 1 and the characteristic
impedance of the bending part 2 can be prevented. As a result, it
is possible to reduce signal reflection as well as a passing loss,
and thus it is possible to reliably and efficiently propagate a
high-frequency signal of an optical communication
transmission-reception module.
In the present embodiment, one high-frequency signal line 4 is
provided to the flexible substrate 1, but a plurality of
high-frequency signal lines 4 may be provided and a DC line may be
provided with diem. The flexible substrate 1 is not limited to a
single layer structure but may have a multi-layer structure. A
plurality of bending parts 2 may be provided and may include the
bending part 2 having an opposite bending direction.
Embodiment 2
FIG. 9 is a perspective view illustrating a flexible substrate
according to Embodiment 2. FIG. 10 is a cross-sectional view of the
flexible substrate in FIG. 9 being bent, which is taken along I-II.
FIG. 11 is a cross-sectional view of the flexible substrate in FIG.
9 being flat, which is taken along III-IV. The width of the
high-frequency signal line 4 is locally small only at the bending
part 2 along I-II.
While the flexible substrate 1 is bent, a thickness d1 of the
dielectric plate 3 of the bending part 2 is smaller than a
thickness d0 of the dielectric plate 3 of the flat part 7
(d1<d0). Accordingly, the capacitance C1 between the
high-frequency signal line 4 and the ground conductor 5 at the
bending part 2 is larger than the capacitance C0 at the flat part 7
(C1>C0).
The characteristic impedance is determined by a ratio of
capacitance and inductance L. The inductance L corresponds to the
line width of the high-frequency signal line 4. Thus, in the
present embodiment, a width w1 of the high-frequency signal line 4
at the bending part 2 is set to be locally smaller than a width w0
at the flat part 7 (w1<w0). Inductance L1 of the bending part 2
is larger than inductance L0 of the flat part 7 (L1>L0). Change
of the local characteristic impedance of the bending part 2
relative to the characteristic impedance of the flat part 7 in the
flexible substrate 1 is compensated by the locally increased
inductance L1 of the bending part 2. Accordingly, discontinuation
between the characteristic impedance Z0 of the flat part 7 and the
characteristic impedance Z1 of the bending part 2 in the flexible
substrate 1 can be solved (Z1.apprxeq.Z0). An optimum value needs
to be selected as the width w1 of the high-frequency signal line 4
at the bending part 2 in accordance with the change amount of the
thickness of the dielectric plate 3 due to bending, the relative
permittivity of the dielectric plate 3, the bending angle, the
frequency of a propagating high-frequency signal, and the like.
Embodiment 3
FIG. 12 is a perspective view illustrating a flexible substrate
according to Embodiment 3. FIG. 13 is a cross-sectional view of the
flexible substrate in FIG. 12 being bent, which is taken along
I-II. FIG. 14 is a cross-sectional view of the flexible substrate
in FIG. 12 being flat, which is taken along III-IV.
Similarly to Embodiment 1, the absent part 6 facing the
high-frequency signal line 4 is locally provided on the ground
conductor 5 only at the bending part 2. Ground conductors 8 are
locally provided on sides of the high-frequency signal line 4 only
at the bending part 2. The ground conductor 5 and each ground
conductor 8 are electrically connected with each other through a
via 9 penetrating through the dielectric plate 3.
The high-frequency signal line 4 and the ground conductor 5 form a
micro strip line at the flat part 7. The high-frequency signal line
4 and the ground conductors 8 form a coplanar line at the bending
part 2.
At the bending part 2, the thickness of the dielectric plate 3 is
small but the absent part 6 is provided, and thus, influence of the
thickness of the dielectric plate 3 on the characteristic impedance
of the bending part 2 is negligible. A characteristic impedance Z3
of the bending part 2 included in the coplanar line is determined
by a width w3 of the high-frequency signal line 4 of a planar line,
an interval s3 between the high-frequency signal line 4 and each
ground conductor 8, and the permittivity of the dielectric plate 3.
Influence of the permittivity of the dielectric plate 3 is dominant
at a surface layer part.
Thus, a distance s3 between the high-frequency signal line 4 and
each ground conductor 8 is set to be sufficiently smaller than the
distance between the high-frequency signal line 4 and the ground
conductor 5. Accordingly, change of the local characteristic
impedance of the bending part 2 relative to the characteristic
impedance of the flat part 7 in the flexible substrate 1 is
compensated. Thus, discontinuation between the characteristic
impedance of the flat part 7 and the characteristic impedance of
the bending part 2 in the flexible substrate 1 can be solved.
In addition, since the ground conductors 8 are provided, a bent
shape can be easily maintained. Thus, a stable high frequency
characteristic can be obtained against external force at
installation or stress on the flexible substrate 1 due to
temperature change or the like.
Optimum values need to be selected as the dimensions of the absent
part 6, the width w3 of the high-frequency signal line 4 of the
coplanar line, the interval s3 between the high-frequency signal
line 4 and each ground conductor 8, and the like in accordance with
the change amount of the thickness of the dielectric plate 3 due to
bending, the relative permittivity of the dielectric plate 3, the
bending angle, the frequency of a high-frequency signal
propagating, and the like.
FIG. 15 is a perspective view illustrating a modification of the
flexible substrate according to Embodiment 3. A ground conductor 8
forming a coplanar line is provided only on one side of the
high-frequency signal line 4. In actual wiring, a plurality of
high-frequency signal lines 4 are provided in a limited space in
some cases. In such a case, when the ground conductor 8 is provided
only on one side of the high-frequency signal line 4, space saving
can be achieved, and constraints on disposition of other wires can
be reduced. Other configurations and effects are same as those of
Embodiment 3.
Embodiment 4
FIG. 16 is a perspective view illustrating a flexible substrate
according to Embodiment 4. FIG. 17 is a cross-sectional view taken
along I-II in FIG. 16. FIG. 18 is an enlarged cross-sectional view
of the bending part of the flexible substrate according to
Embodiment 4 being bent. A recess 10 facing the high-frequency
signal line 4 is locally provided on the second main surface of the
dielectric plate 3 only at the bending part 2. The ground conductor
5 is embedded in the recess 10.
When the flexible substrate 1 is bent, a length l1 of the
high-frequency signal line 4 outside the bending part 2 largely
extends as compared to a length l2 of the ground conductor 5 inside
the bending part 2. When the bending part 2 has a small steepness
and the bending part 2 has a relatively large bending region,
influence of the extension of the high-frequency signal line 4 on
the characteristic impedance is larger than influence of decrease
of the thickness of the dielectric plate 3. Thus, contrariwise, the
characteristic impedance may increase due to increase of the
inductance at the bending part 2 in some cases.
However, in the present embodiment, the recess 10 is provided at
the bending part 2 and embedded in the ground conductor 5.
Accordingly, the distance between the high-frequency signal line 4
and the ground conductor 5 is shorter and the capacitance is larger
at the bending part 2. With this structure, change of the local
characteristic impedance of the bending part 2 relative to the
characteristic impedance of the flat part 7 in the flexible
substrate 1 is compensated. Thus, discontinuation between the
characteristic impedance of the flat part 7 and the characteristic
impedance of the bending part 2 in the flexible substrate 1 can be
solved.
Optimum values need to be selected as the depth, size, and the like
of the recess 10 in accordance with extension of the high-frequency
signal line 4 due to bending (l1-l2), the change amount of the
thickness of the dielectric plate 3 due to bending, the relative
permittivity of the dielectric plate 3, the bending angle, the
frequency of a propagating high-frequency signal, and the like.
Embodiment 5
FIG. 19 is a perspective view illustrating a flexible substrate
according to Embodiment 5. A dielectric 11 having a permittivity
different from the permittivity of the dielectric plate 3 is
locally embedded in the dielectric plate 3 only at the bending part
2 in a region facing the high-frequency signal line 4. The second
main surface of the dielectric plate 3 provided with the ground
conductor 5 is dug to embed the dielectric 11, but the first main
surface provided with the high-frequency signal line 4 may be dug
to embed the dielectric 11.
When the characteristic impedance of the bending part 2 decreases
due to bending, a material having a relative permittivity smaller
than the relative permittivity of the dielectric plate 3 is
selected as the material of the dielectric 11. When the
characteristic impedance of the bending part 2 increases due to
bending, a material having a relative permittivity larger than the
relative permittivity of the dielectric plate 3 is selected as the
material of the dielectric 11. With these structures, change of the
local characteristic impedance of the bending part 2 relative to
the characteristic impedance of the flat part 7 in the flexible
substrate 1 is compensated. Thus, discontinuation between the
characteristic impedance of the flat part 7 and the characteristic
impedance of the bending part 2 in the flexible substrate 1 can be
solved.
Optimum values need to be selected as the relative permittivity of
the dielectric 11, the embedding depth of the dielectric 11, the
size of the dielectric 11, and the like in accordance with the
change amount of the thickness of the dielectric plate 3 due to
bending, the relative permittivity of the dielectric plate 3, the
bending angle, the frequency of a propagating high-frequency
signal, and the like.
REFERENCE SIGNS LIST
1 flexible substrate; 2 bending part; 3 dielectric plate; 4
high-frequency signal line; 5 ground conductor; 6 absent part; 8
ground conductor, 10 recess; 11 dielectric
* * * * *